"with this new rule imposed without prior notice."

It's not a 'new rule', it is just a change in the terminology that Amazon is using to show different categories/levels of what is being held.

Amazon made the decision a couple years ago that they would lend YOUR money out by not collecting for sales immediately. It started out as a 'net 30' transaction. When Amazon saw how many more business sales they were getting, they did what Amazon does best -- they expanded the terms for even more time with no interest to be earned by the sellers! While they say payments usually occur within 30 to 45 days, there have been various Forum posts about some going 90 days.

An explanation of the terminology can be found here but no date given for when the split in in the reporting language happened.

https://sellercentral.amazon.com/help/hub/reference/GLRNLZDAKHDBBVBY

What is a deferred transaction?

A deferred transaction is a transaction that will be paid out to you at a future date. Most transactions are deferred for some period of time before they are paid out.

You can view a list of your deferred transactions on the Transaction view page. On this page, select "Transaction Status" as Deferred transactions from the dropdown, and click Update to see deferral reasons and the expected payment release date for all deferred transactions. After the payment release date, transaction status will update to Released. Note that only released transactions are included for payout in a settlement period.

Why is my transaction deferred?

A transaction is deferred while an order is awaiting delivery or pending buyer payment.

The following are the deferral reasons you may see on the Transaction view page:

Delivery date policy (Orders are awaiting delivery)

Orders placed by customers on Amazon are typically subject to Delivery date-based reserve policies. Sales proceeds are reserved until we know the buyer received the order in the promised condition, and to ensure that you have enough funds to fulfill any returns, claims, or chargebacks.

Note: Previously, sales proceeds withheld through Delivery Date-Based Reserves appeared as a lump sum amount "Account-Level Reserves" on your Payments Dashboard. This is now shown as deferred transactions so that you can know exactly which orders are subject to these policies and for how long they will be reserved.

Invoiced orders (Orders are pending buyer payment)

Invoiced orders placed by Amazon Business customers are deferred while awaiting payment by the buyer. These transactions will be released after the customer completes their invoice payment, which typically occurs within 30-45 days after the order date. Note that these orders are also subject to delivery date policies, but in most cases, the invoice due date will be later than the reserve period.